SSIA. Not sure if it's xinitrc, but it's the likely culprit so far.
It looks like this is gdm:
great-sage-equal-to-heaven:~% sudo lsof | grep xses
gdm-binar 3761 root 11r REG 253,0 4271 3102181
In addition it looks like the file gets opened in gdk/daemon/slave.c
apparently when gdm for some reason can't get an fd for the log file.
should be fixed in rawhide soon.